Hydrologic interactions between plants and soils in shrub-dominated arid ecosystems: Effects of global environmental change

Global environmental change may greatly affect water availability and the ability of plants to use water. Because plant productivity in arid and semi-arid ecosystems is highly limited by water and because responses to changes in water availability will likely differ among species, global environmental change may ultimately cause changes in plant community species composition and ecosystem function. The interaction of three global environmental change factors important to arid ecosystems—invasions of alien species, rising levels of atmospheric CO₂, and increased precipitation—has been hypothesized to lead to an acceleration of the natural fire cycle in arid shrublands, leading to large-scale conversions of native shrub-steppe plant communities to post-fire communities dominated by herbaceous annual species. However, the consequences of large-scale, wildfire-driven conversion of native perennial shrub communities to post-fire communities dominated by invasive annual species on hydrology and carbon cycling are poorly understood.; Thus, the objectives of this research were (1) to quantify how such fire-induced changes in plant community structure affect ecosystem hydrology (temporal and spatial patterns of soil water storage and recharge, water vapor fluxes, and the lateral distribution of surface water) and carbon cycling (ecosystem CO₂ fluxes); (2) to quantify how rising CO ₂ may determine the responses of native desert shrubs to expected reductions in soil resource heterogeneity; and (3) to experimentally estimate how resultant simplification of plant canopies (e.g., surface roughness) may impact evapotranspiration and the relative contributions of soil evaporation and plant transpiration.; The results of these studies conducted in a sagebrush ecosystem and an adjacent post-fire ecosystem in the western Great Basin in the second and third year after fires indicate that fires and post-fire plant succession may (1) reduce soil water recharge in years with normal winter precipitation (snows); (2) decrease the amount of plant-available water in the soil profile during phenologically important times of the years, but will not affect soil water storage in years when winters are very dry; (3) have little effect on ecosystem carbon balance in dry years; (4) reduce the lateral heterogeneity of soil water via elimination of shrub and inter-shrub patterns, which (5) possibly precludes seedling establishment of patch-dependent native perennials (sagebrush). (6) Rising atmospheric CO₂ may compensate to some degree for these possible impediments to native shrub seedling establishment by accelerating seedling root growth and stimulating greater investment of seedling roots in deeper soil layers and thereby enhancing water acquisition during early stages of seedling development. Overall, results from these studies indicate that a complex set of interactions among global environmental change factors have the potential to alter the structure and function of arid shrubland ecosystems.